Is it a warning or an error? What exactly does it say? 

Thank you for choosing TurboTax.

That looks like a Warning and not an Error. If it is a Warning and not an Error, then it can be ignored and skipped. The Warning alert does not automatically mean something was done or entered incorrectly. Click on Review to verify, by clicking on "Applies to me". Once reviewed you can click "Done with warnings" and return to your review. 

Warnings are there just to ensure that if the situation applies to you, you can go back and make any necessary modifications.
Errors, on the other hand, will prevent you from filing because something in your return is incorrect and the return will not be accepted by the Canada Revenue Agency.

Thank you for choosing TurboTax.

I have already said I am "done with warnings."

But when I try to netfile, I get: We're sorry, the Canada Revenue Agency was unable to accept your NETFILE tax return", based on the OAS error.

Can you please go to the left side menu of your return and click on Income- T-Slips-T-Slip Summary and make sure that you did not accidentally check off the box that says T4A(OAS)?

If you are still not able to resolve this issue, please contact our phone support team at 1-888-829-8608 for further assistance.